Bindings and Refs

In certain situations, Roact's reconciliation process is ill-suited for managing some Instance properties. For cases like this, Roact provides escape hatches in the form of Bindings and Refs.

Bindings and Refs are intended to be used in cases where Roact cannot solve a problem directly, or its solution might not be performant enough, like:

  • Invoking functions on Roblox Instances
  • Dynamically resizing a host component to fit its contents
  • Gamepad selection
  • Animations

Bindings

Bindings are special objects that Roact automatically unwraps into values. When a binding is updated, Roact will change only the specific properties that are subscribed to it.

Binding Properties

Bindings can be used to provide an external source for a prop value, or to update those values outside of the Roact reconciliation process.

First, create a binding and an updater using Roact.createBinding() and put the results somewhere persistent. createBinding returns two results: a binding object and an updater function, which is used to update the binding's value.

local Foo = Roact.Component:extend("Foo")

function Foo:init()
    -- createBinding takes an initial value; for our purposes, 0 is fine
    self.clickCount, self.updateClickCount = Roact.createBinding(0)
end

Then, connect the binding value to something that we're rendering and the updater to something that will invoke it.

function Foo:render()
    return Roact.createElement("TextButton", {
        -- Roact unwraps the binding, sets the Text property to the binding's value,
        -- and subscribes to the binding
        Text = self.clickCount,
        [Roact.Event.Activated] = function()
            -- When the user clicks the button, the count will be incremented and
            -- Roact will update any properties that are subscribed to the binding
            self.updateClickCount(self.clickCount:getValue() + 1)
        end
    })
end

The result of this example is a TextButton that displays the number of times it's been clicked. In this case, we connect the updater to the button's Activated event. Other use cases could be connecting it to some external property in didMount or passing it to a child component as a callback.

Mapped Bindings

Often, a binding's value isn't useful by itself. It needs to be transformed into some other value in order to be useful when assigned to an Instance property.

Let's modify the above component to make use of a mapped binding:

function Foo:render()
    return Roact.createElement("TextButton", {
        -- Roact will receive the mapped value from the binding
        Text = self.clickCount:map(function(value)
            return "Clicks: " .. tostring(value)
        end),
        [Roact.Event.Activated] = function()
            -- When the user clicks the button, the count will be incremented
            self.updateClickCount(self.clickCount:getValue() + 1)
        end
    })
end

Our mapped binding transforms the number of clicks into a string. Now the TextButton will display "Clicks: 0" instead of just the number!

Refs

While bindings are most helpful for individual props, we often want to access an entire Roblox Instance and its methods.

Refs are a special type of binding that point to Roblox Instance objects that are created by Roact.

Refs can only be attached to host components. This is different from React, where refs can be used to call members of composite components.

Refs in Action

To use a ref, call Roact.createRef() and put the result somewhere persistent. Generally, that means that refs are only used inside stateful components.

local Foo = Roact.Component:extend("Foo")

function Foo:init()
    self.textBoxRef = Roact.createRef()
end

Next, use the ref inside of render by creating a host component. Refs use the special key Roact.Ref.

function Foo:render()
    return Roact.createElement("TextBox", {
        [Roact.Ref] = self.textBoxRef,
    })
end

Finally, we can use the value of the ref at any point after our component is mounted.

function Foo:didMount()
    -- The actual Instance can be retrieved using the `getValue` method
    local textBox = self.textBoxRef:getValue()

    print("TextBox has this text:", textBox.Text)
end

Refs as Host Properties

In addition to providing access to underlying Roblox objects, refs also provide a handy shortcut for Roblox Instance properties that expect another Instance as their value. One commonly-encountered example is NextSelectionLeft and its counterparts.

Roact's Roblox renderer knows that bindings are not valid Roblox Instance values, so it will unwrap them for you:

local Bar = Roact.Component:extend("Bar")

function Bar:init()
    self.leftButtonRef = Roact.createRef()
    self.rightButtonRef = Roact.createRef()
end

function Bar:render()
    return Roact.createElement("Frame", nil, {
        LeftButton = Roact.createElement("TextButton", {
            [Roact.Ref] = self.leftButtonRef,
            NextSelectionRight = self.rightButtonRef,
        }),
        RightButton = Roact.createElement("TextButton", {
            [Roact.Ref] = self.rightButtonRef,
            NextSelectionLeft = self.leftButtonRef,
        })
    })
end

Since refs use bindings under the hood, they will be automatically updated whenever the ref changes. This means there's no need to worry about the order in which refs are assigned relative to when properties that use them get set.

Function Refs

The original ref API was based on functions instead of objects (and does not use bindings). Its use is not recommended for most cases anymore.

This style of ref involves passing a function as the Roact.Ref prop as opposed to a dedicated ref object:

local function Baz(props)
    return Roact.createElement("TextBox", {
        [Roact.Ref] = function(instance)
            -- Be careful to guard against nil refs; this is a gotcha of
            -- function refs.
            if instance ~= nil then
                print("TextBox has this text:", instance.Text)
            else
                print("TextBox ref removed.")
            end
        end,
    })
end

Warning

Function refs, unlike bindings, cannot be used as properties for host components. This will result in an error.

Warning

When a function ref is called, it's not guaranteed that its sibling or parent components have finished mounting. Causing side effects here can cause difficult-to-trace bugs.

Warning

When a component with a function ref unmounts, or when the ref value changes, the component's ref is passed nil.